I picked up a 1904/39 Mauser this past weekend in very good condition, smooth and tight action, barrel is bright with sharp rifling. This is my first experience with the 8x57 round, from what I've read, the 8x57 comes in 2 variants, the one uses a .318 bullet, the other a .323

Does anyone know which is the proper bullet for this rifle?

Thanks!

Not familiar with an 04/39 Mauser. Is this perhaps the Portugese Veriguero converted to 8x57? If so, they were .323 or "S" bores.

If in doubt, one could slug the barrel or have a gunsmith do it and verify the bore diameter. I suspect it is probably a .323". Most U.S. factory ammo for the 8mm Mauser is underloaded to accommodate smaller bore diameter rifles. All the best...
Gil

Charley, yes, its the converted Portuguese Veriguero. Thanks for the information on that. Would there be a possibility of damaging the rifle if I did shoot the .318 out of a .323? And do you know if most of the american manufactured is in the .323, or is that going to be hard to find?

I see the 8x57J is .318, and the .323 is 8x57JS, I'm guessing you mean the JS as being the S. I looked at boxes of both Remington and Winchester rounds and did not see any marking differentiating the rounds from J and JS.

Again, thanks for all your help. First time moving away from .22, want to make sure I do it right

The only "J" bores you are likely to run into are "liberated" sporters from Germany. There might be a few G or K88s that were not converted to "S", but they are mostly in collections and are not active shooters. It is a pretty reasonable statement to say that all WWI and later military rifles in 8x57 are "S" bores, but caution is good.
US 8x57 is very, very mild, so you aren't going to have a catastophic failure even if it went into a "J" bore.
Best bet is to slug the bore and throat of an unknown 8x57 rifle, and measure carefully!

The Portugese converted the Verigueros in the late thirties, hence the '39' model designation. By that time, all military 8x57s were "S" bores.